Interpretive Summary: Airborne movement of aerially applied crop protection or public health sprays is generally quantified using active droplet collectors, such as a rotating microscope slide. The efficiencies with which these collectors capture spray droplets are a function of the sampler’s rotational speed, the spray droplet size being sampled, and the wind speed in which the spray is entrained. New evaluation protocols were developed to determine the collection efficiencies of two rotary impaction devises as a function of wind speed and drop size in a low-speed wind tunnel. Recommendations are presented for correcting field measured volumetric deposition and spray droplet size data based on the sampler’s collection efficiencies. The results will help researchers and public health personnel to have a better understanding of these samplers and to better interpret spray efficacy data from spray applications.

Technical Abstract:
Measurements of spray volume and droplet size are critical to evaluating the movement and transport of applied sprays associated with both crop production and protection practices and vector control applications for public health. Any sampling device used for this purpose will have an efficiency of collection that is a function of the sampling device itself, the droplet size of the spray being sampled, and the airspeeds under which the sampling is conducted. This study focuses on two rotary impaction devices, the Hock and the FLB samplers, that were evaluated under two droplet sized sprays and four airspeeds. The collected spray concentrations were compared to standard passive samplers whose theoretical collection efficiency was calculated and used to estimate the actual spray volume sampled. Additionally, droplet sizing information derived from image analysis of droplet deposits on the rotary impactor collection surfaces was compared to actual measurements of droplet size of the sampled spray cloud. Generally, overall collection efficiencies ranged from 2.5 to 20% with the FLB being more efficient than the Hock and with lower efficiencies at higher airspeeds for both samplers. Comparison of the droplet sizing data showed that the FLB sampler tended to under predict the DV10 and DV50 data while the Hock tended to over predict the DV90 data.